Can't go to therapy, or afford it.

I can't go to a therapist because my parents don't believe in depression and they're ashamed by me, and think I'm just weak minded. Its also too expensive. My sister understands what I'm going through but she says I have to wait till I'm 18 to do anything. I feel like by that time I'll have either killed myself or failed in high school.

How can I cope on my own?